Power and Cooling Calculations for a 4-GPU Rack in Muscat's Climate

A four-GPU sovereign AI rack in Muscat is not the same engineering problem as the same rack in Frankfurt. Outdoor air bakes the building shell, salt-laden coastal humidity attacks fin stacks, and dust from the Wadi Al Kabir corridor coats every filter inside a quarter. This piece walks through the heat math for a 4x H100 SXM5 node, compares cooling topologies for that load, and lays out a defensible UPS plan that respects the realities of the Omani grid.

Muscat ambient: 35 to 48 C peak, dust, and sea-air corrosion

Muscat is a hot-arid coastal site, and any data hall design must respect three local stressors:

• Dry-bulb temperature. Average summer afternoons sit at 38 to 42 C. Heatwaves, especially in June and July, push to 45 to 48 C in inland districts like Al Amerat and Bawshar. Design outdoor lookup tables around a 45 C sustained worst case, not the climatology mean.
• Dust loading. Wadi runoff and shamal winds drive PM10 well above the WHO interim target several days a year. MERV 13 pre-filters with quarterly replacement are the floor, not the ceiling, for any AI room within five kilometres of the coast or a wadi.
• Sea-air corrosion. Coastal Muscat sites within two kilometres of the shoreline experience chloride-driven attack on aluminium fin stacks and copper coils. Specify epoxy-coated condenser coils and stainless louvres, and inspect the heat-rejection plant every six months.

The relevant thermal target inside the white space is the ASHRAE A2 thermal envelope, which permits cold-aisle inlets between 10 and 35 C with up to 80 percent relative humidity. We design Hosn deployments to the tighter recommended band of 22 to 27 C inlet, which trades a little operating cost for substantially better long-term silicon health and lower fan power.

Per-rack heat math for a 4x H100 SXM5 node

The starting point is silicon-level Thermal Design Power. NVIDIA publishes the H100 SXM5 at 700 W under sustained load (NVIDIA H100 datasheet). The bill of heat for a single HGX node looks like this:

• 4 x H100 SXM5 GPUs at 700 W each: 2,800 W
• 2 x host CPUs (e.g. Sapphire Rapids 56-core), nominal 350 W each: 700 W
• NVMe storage, DRAM, baseboard, fans: ~250 W
• Top-of-rack switching (1 x 400 GbE leaf): ~150 W
• PSU conversion losses at 92 percent efficiency: ~330 W

That sums to roughly 4.2 kW peak draw at the PDU, of which about 3.6 kW becomes heat that must leave the rack. In imperial cooling units, 3.6 kW equals approximately 12,280 BTU per hour. A typical empty 42U rack adds negligible thermal mass, so plan the cold-aisle CFM around the GPU node alone. Cisco and Schneider Electric guidance on data center hot climate cooling recommends roughly 160 CFM per kW of IT load at a 12 C delta, so this rack needs about 580 CFM of supply air to stay in band.

Cooling options: CRAC, CRAH, and rear-door heat exchangers

Three topologies cover the realistic envelope for a single 4-GPU sovereign rack in Muscat:

1. Perimeter CRAC (DX). A 5 to 7 ton direct-expansion unit on the room perimeter, feeding under a raised floor or via overhead ducts into a contained cold aisle. Lowest capital cost, simplest operations, but least efficient at part load. Fine for a single rack with one or two future expansion slots.
2. CRAH on chilled water. If the host building already runs a chilled-water plant (most ministry HQs and bank data centres do), a CRAH unit consuming 10 C supply water gives 30 to 40 percent better PUE than DX. Worth the integration work above 8 to 10 kW per rack or when chilled water is already free at the wall.
3. Rear-door heat exchanger. A passive or fan-assisted coil mounted on the rear of the rack, fed by the chilled-water loop, removes heat at the source so the room itself stays at building-ambient. Best for dense compute (above 15 kW per rack) and retrofits where there is no room to contain aisles. Overkill for a single 3.6 kW node, but it is the right answer when you grow to four nodes in one row.

For most Hosn Tower deployments we default to perimeter DX with cold-aisle containment, then revisit at the second rack. Hot-climate ASHRAE technical committees specifically warn against open-room cooling for any AI workload above 2 kW per rack: bypass air and recirculation murder both PUE and silicon longevity in 45 C summer conditions.

UPS sizing and redundancy for the Omani grid

Mainland Oman is supplied by the OETC transmission network, which is generally stable but does experience sub-second voltage sags during summer peak loading and rare multi-second outages from substation events. Sovereign AI workloads cannot tolerate even one of those events without protection. The sizing rule we use:

• Capacity. Total IT load 3.6 kW + cooling overhead in the same protected envelope. For a single rack we target a 6 kVA to 8 kVA double-conversion UPS at 0.9 power factor, leaving ~30 percent headroom for transients and future GPU drift.
• Runtime. 10 to 15 minutes at full load. That is enough for a properly maintained diesel generator to start, stabilise, and assume the load. Do not chase 30+ minute battery runtimes; chase a working generator with 24 hours of fuel and a quarterly test schedule.
• Topology. Double-conversion online, never line-interactive, for AI hardware. Add an N+1 module for any rack feeding a regulator-, defence-, or banking-tier workload.
• Power quality. Pair the UPS with a static transfer switch and a generator with proper synchronisation. Generator-only protection misses the sag events that actually corrupt training runs.

Battery chemistry matters in Muscat: lithium iron phosphate keeps capacity at the 30 to 35 C UPS-room ambient that is realistic in this climate, where VRLA lead-acid degrades fast. The capex premium pays back in three to four years on Muscat sites.

Heat math, cooling topology, and UPS sizing are three of the answers the broader AI rack power cooling airgap playbook expects every sovereign buyer to have ready before signing a hardware order. Email [email protected] for a one-hour briefing where we walk your facilities team through the room-level numbers for your specific site.